Derivative Instruments and Hedging Activities - Fair Value of Derivative Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Sep. 30, 2018	Mar. 31, 2018
Derivative assets:
Total derivative assets — gross	$ 90.8	$ 232.5	$ 80.5
Gross amounts offset in the balance sheet	(35.8)	(34.3)	(31.5)
Cash collateral received	0.0	(12.2)	(0.4)
Total derivative assets — net	55.0	186.0	48.6
Derivative liabilities:
Total derivative liabilities — gross	(83.3)	(58.8)	(85.4)
Gross amounts offset in the balance sheet	35.8	34.3	31.5
Total derivative liabilities — net	(47.5)	(24.5)	(53.9)
Derivatives subject to PGC and DS mechanisms:
Derivative assets:
Total derivative assets — gross	1.3	3.0	0.9
Derivative liabilities:
Total derivative liabilities — gross	(0.2)	(0.1)	(0.6)
Derivatives designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	7.1	2.4	0.3
Derivative liabilities:
Total derivative liabilities — gross	(5.7)	(1.4)	(8.1)
Derivatives designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	7.1	1.5	0.3
Derivative liabilities:
Total derivative liabilities — gross	0.0	(0.4)	(4.0)
Derivatives designated as hedging instruments: | Cross-currency contracts
Derivative assets:
Total derivative assets — gross	0.0	0.9	0.0
Derivative liabilities:
Total derivative liabilities — gross	0.0	0.0	(2.2)
Derivatives designated as hedging instruments: | Interest rate contracts
Derivative liabilities:
Total derivative liabilities — gross	(5.7)	(1.0)	(1.9)
Derivatives not designated as hedging instruments:
Derivative assets:
Total derivative assets — gross	82.4	227.1	79.3
Derivative liabilities:
Total derivative liabilities — gross	(77.4)	(57.3)	(76.7)
Derivatives not designated as hedging instruments: | Foreign currency contracts
Derivative assets:
Total derivative assets — gross	22.7	19.1	13.6
Derivative liabilities:
Total derivative liabilities — gross	(4.9)	(14.0)	(39.5)
Derivatives not designated as hedging instruments: | Commodity contracts
Derivative assets:
Total derivative assets — gross	59.7	208.0	65.7
Derivative liabilities:
Total derivative liabilities — gross	$ (72.5)	$ (43.3)	$ (37.2)